1. Field of the Invention
The present invention relates to an image reader using a contact image read sensor, which gathers two-dimensional document image data by scanning an image on an original document with an image read unit having an array of one-dimensional image read elements, with the translating motion of the read unit relative to the original image. More particularly, the invention relates to an image reader capable of properly handling the out-of-focus caused by the irregularities and coming-up of the document surface, and an image reading method for the image reader.
2. Discussion of the Related Art
In some of the image readers each using a contact image read sensor, a one-dimensional rod lens array is used for the image-forming optical system. The image reader of this type in which a one-dimensional rod lens array is used for the image forming optical system, will be described with reference to FIG. 10 showing an external view of a conventional image reader.
The image reader, as shown in FIG. 10, is made up of an image read unit 1 having an image sensor for reading an image from an original document, a drive mechanism 2 for moving the read unit 1, a drive shaft 3 for allowing the movement of the read unit 1 by the drive mechanism 2, an image processor board 4 having an image processor circuit thereon for processing image signals from the image sensor of the read unit 1 for the gray level adjustment, tone characteristic correction, digitizing the image signals as required, and the like, and a platen cover 5 to be set on an original document located on a platen glass to keep it flat.
The original, held by the platen cover 5, is scanned by the image read unit 1, to read out the image of one line. The read unit 1 is rectilinearly moved in the slow scan direction under control of the drive mechanism 2. Subsequently, the image of the next line is read out of the document surface. The image signals read out by the read unit 1 are processed in the form of read image data by the image processor board 4.
The details of the construction of the image read unit 1 will be described with reference to FIG. 11. FIG. 11 is a sectional view showing a conventional read unit of the image reader of FIG. 10.
The conventional image read unit 1 includes an optical system and an image read system. The optical system is made up of a light projector 11, contained in a carriage case 10, for projecting light onto an original document 7 located on a platen glass 6, and a rod lens array 12, consisting of an array of rod lenses, for receiving an image of reflected light from the original document 7 and forming an image of the reflected light on a sensor array 13. The image read system is made up of the sensor array 13 which consists of an array of photo sensors, a sensor array mounting board 14 having the sensor array 13 mounted thereon, a signal processor board 15 for reading image signals output from the sensor array 13 and time-sequentially outputting the image signals to the image processor board 4, and an IR cut filter 16, located between the optical system and the read system, for cutting off infrared rays.
The original document 7 is positioned in place by the platen cover 5 and the platen glass 6. To this position of the original document 7, the focal position of the optical system is adjusted in connection with the relative positions of the surfaces of the rod lens array 12 and the sensor array 13, in a factory.
The depth of field of the rod lens array 12, which is used in association with the contact sensor array 13, is approximately 1 mm at most. This figure is insufficient for the required depth of field of the image reader. The image-bearing surface of the original document to be placed on the platen glass is not always flat. The stitched portion of a spread book is not flat. Some documents, such as documents having three-dimensional objects bonded thereto and documents having creases, have uneven surfaces. When this type of document is set in the image reader of the fixed focal position, and an image thereon is read out by the image reader, the readout image is partially blurred. Accordingly, an unsatisfactory readout image can be obtained.
To cope with the out-of-focus problem, attempts to improve the rod lens array have been made. The results of the improvement are still unsatisfactory. When considering the fact that the depth of field of the rod lens array is shallow, there is possibly a theoretical limit in improving the out-of-focus problem of the image reader of the type in which the rod lens array is used for an imaging optical system.
In connection with this problem, another proposal has been made, in which the focal position is directly varied in synchronism with the image read operation, thereby obtaining a better image. In the image reader disclosed in Japanese Patent Unexamined Publication No. Sho. 59-36477 (conventional example 1), one scan line is divided into 10 number of segmental scan lines. The readout signal is sampled and held to form a histogram. The histogram is used for obtaining exact image data.
In the optical information reproducing system disclosed in Japanese Patent Unexamined Publication No. Hei. 4-40746 (conventional example 2), the best focal position is secured by moving the read means of the image sensor along the path of the light beam reflected from the surface of an original document. To this end, predetermined lines are provided on both ends of the document. The read means is controlled using the read signals of the lines.
In the image reader of the conventional example 1, a read head is vertically vibrated at frequency of 10 KHz for one scan line of 0.2 mm width. This cannot satisfy the pixel density (16 lines/mm to 32 lines/mm) required at the present stage. The image is picked up in a sample/hold manner. When the vertical motion for image reading is divided by 10, read elements must be operable at a speed 1/10 as high as a normal speed, viz., a high speed multiplied by the dividing number. It is very difficult to realize such read elements by the present technology.
In the optical information reproducing system of the conventional example 2, the reference lines must be provided on the readout image. This restricts the image reading operation. The control for the focal position adjustment properly functions in the image area including the reference lines extending in the slow scan direction. In the fast scan direction, only the fixed reading operation functions. Accordingly, when reading an original document of which the surface is irregular in the central portion, the readout image is unsatisfactory in quality.